When working with metals, it is essential to consider all their properties, especially hardness. This characteristic is one of the most valued and appreciated in metals. Depending on the intended use, knowing the hardness of a metal will help us choose the most suitable one for each purpose.
The importance of its correct measurement so that the projects in which it is a part function smoothly requires that it undergo certain tests. All of them will provide us with information about, among other things, its hardness, which will be the focus of today’s article, so we will see what types of tests are usually performed and how we can calculate it.
What is hardness?
Hardness is the physical property of materials that refers to their resistance to being deformed, scratched, or penetrated. It can be measured in several ways, although it all depends on the type of material and the context in which it is being used. If we talk about metals, which is the material we specialize in at Initube, hardness can be evaluated through tests such as the Mohs, Rockwell, or Brinell scales (which we will explain later). Hardness is important in many areas of its production, from the manufacture of tools to the selection of materials for specific applications, such as the construction of structures or the manufacture of medical devices.
Before continuing, it is necessary to make a brief clarification to explain the term ‘technical hardness‘, as it is often confused or, on the contrary, believed to be the same. In reality, it refers to the ability of a material to resist permanent deformations or penetrations when subjected to mechanical stresses (compression, bending, traction, or abrasion) in a technical or engineering environment.
Technical hardness measures the ability of a material to withstand loads and resist wear under specific usage conditions. This differentiates it from hardness as resistance to deformations. However, the same scales can be used to measure both, providing numerical values that indicate the relative resistance of the material to deformation or penetration. The choice of scale and approach will depend on the type of material, its application, and the specific design and manufacturing requirements.
This quality of metals is also very important and appreciated in engineering because it directly affects the ability of a material to withstand loads, resist wear, and maintain its structural integrity under various service conditions. Engineers use this information to select suitable materials for specific applications, design components that meet strength and durability requirements, and prevent premature failures due to deformation or fatigue.
Why do hardness tests?
This type of test is indispensable in the field of materials research, in engineering, or from a more technical perspective, because it provides many answers and fulfills certain objectives:
Material differentiation
Tests help us distinguish between different types of materials and thus select the most suitable one for a specific application.
Material analysis and development
They are also fundamental tools in the analysis, development, and improvement of materials and technologies. They provide crucial information about their properties and behavior under different conditions to which they have been subjected.
Quality control
The hardness values determined during tests are important for quality control in the industry. They help ensure that all products meet the required standards and specifications, both at the input and output of manufacturing processes.
Material differentiation
In cases where material mixtures are handled, both with similar compositions or without them, tests help differentiate them to maintain the quality and integrity of the final products.
Damage analysis
Finally, they are also used in damage analysis to understand the causes of failures or deterioration in materials. They provide information about the strength and durability of materials under various service conditions to understand when they are weakest or can be improved.
How is it measured?
As we briefly mentioned above, there are different tests or methods that can be performed and chosen to obtain hardness measurements:
Rockwell
This method is chosen when the materials to be analyzed have been machined or rolled and have a smooth surface. Various scales can be applied, which are determined by the combination of different indenters and loads.
Brinell
The Brinell test is suitable for any cast piece or product, regardless of whether the surface is rough. In this method, standard test loads of 3000 kg are used for steels and loads of 500 kg for aluminum, copper, and bronze.
Vickers and Knoop
Also known as microhardness tests, Vickers and Knoop have a special application, as they are used when Rockwell or Brinell methods cannot be used because the materials are very thin or small. In these methods, the test load is a maximum of 1000 g, with the most common being 100 g, 200 g, and 500 g.
Leeb
The Leeb hardness test is a very practical method for field testing, whether on very large parts, structures, machinery, etc. The most common scales are D and G.
Shore
This last test is used to evaluate hardness in plastics, usually carried out with portable equipment. The most common scales are A, B, C, and D, but it always depends on the type of material being tested.
Hardness tests by load
Hardness tests can be classified depending on the load ranges used during the test. That said, it always relies on the magnitude of the main load applied to the sample and is divided into 3 main categories, which allow us to select the most suitable hardness testing method according to the sample size, material hardness, and specific application required:
- Macrohardness. High main loads are used, usually around 5 kgf or more. This way, we obtain proportionally large indentations in the sample. Macro-scale hardness testing methods include Brinell, Vickers, and Rockwell. They are used for materials with higher hardness and for larger parts.
- Low load. The main loads are within the range of 0.2 kgf to 5 kgf. These methods are typically used for small parts, thick layers, and low-hardness materials, with the most common in this category being the Vickers test.
- Microhardness. They require the use of very small main loads, generally less than 0.2 kgf, resulting in very small indentations in the sample. The most common method for microhardness testing is the Vickers method, primarily used to determine the hardness of thin layers, small individual crystals, or inclusions.
Factors that can affect measurement
To obtain accurate results in hardness tests, we need to consider all the factors that can vary the results. We must take into account that if we use small loads, extra attention must be paid to all the elements we will discuss below to ensure the reliability of the results:
External factors
It is essential to control external conditions (light, dirt, vibrations, temperature, humidity, etc.) to prevent them from affecting the measurement process.
Equipment stability
The hardness tester must always be placed on a solid horizontal table. The sample, on the other hand, must be properly secured using a sample holder or anvil to ensure stability during the test.
Indenter positioning
The indenter must be aligned and positioned perpendicular to the surface of the sample being tested.
Consistent lighting
In Vickers, Knoop, or Brinell hardness tests, the lighting setup must be consistent to ensure measurements.
Recalibration of equipment
The hardness tester must be recalibrated or verified each time the indenter or objective lens is changed to maintain measurement accuracy over time.
What to consider
Now that we have explained the ways to measure the hardness of a material, the next question to answer is how to select the most suitable method? We mentioned earlier that it all depends on the material’s requirements. However, we must also consider other elements related to the microstructure and the project’s objective. Therefore, we have made a small summary of everything to pay a little more attention to:
Microstructure of the material
The homogeneity or heterogeneity of this characteristic in the material must be considered. If it is very heterogeneous, it will likely require a larger indentation.
Type of material
The one we choose will, of course, influence the choice of hardness testing method, as some are more suitable for certain types of materials than others.
Size and condition of the piece
Both qualities will affect the choice of method because some of them will be more suitable for larger or smaller parts, as well as for materials in different states (cast, forged, etc.).
Regulations and standards
It is necessary to check if there are specific regulations that require a particular hardness testing method.
Embedding
We need to analyze if the method we choose requires the sample to be prepared by embedding.
Quantity of samples and precision required
We need to consider and know the number of samples to be tested and the precision required for all of them.
Considering these factors, it will be easier to select the most appropriate hardness testing method according to its specific application and obtain reliable and meaningful results. However, if you need help with tests or materials, it is best to consult a professional in the field who can advise you on the most accurate measurement.
At Initube, we believe that specialization helps to achieve perfection, and that is what we have done with the cutting and machining of tubes and profiles of all kinds of ferrous materials. We have accumulated over 40 years of experience in the metallurgical sector. Therefore, if you need a partner who knows about metals, you just need to get in contact with our team so that we can advise you in the best possible way, using our experience.
